The present invention relates generally to active cancellation systems for repetitive phenomena, and more specifically to a fast adapting, low-cost solution to this problem.
Linear flow, air duct systems, for example, Chaplin U.S. Pat. No. 4,122,303; Warnaka, U.S. Pat. No. 4,473,606 and Eriksson U.S. Pat. Nos. 4,677,676 and 4,677,677, take advantage of directional flow in linear, one dimensional flow to utilize an upstream sensor, followed by a cancellation actuator and downstream error sensor in sequence. These systems cancel repetitive and random noise. Chaplin characterizes the controller as a general convolution process, including a "programme of time-related operational steps." Warnaka uses adaptive filters to speed adaptation time and allow greater spacing between the speaker and the duct. Eriksson specifies recursive least mean square (RLMS) and least mean square (LMS) adaptive filters to perform the convolutions and measure the system transfer functions in the presence of noise.
These systems fail to utilize external synchronization timing to provide selective cancellation of repetitive Phenomena in one to three dimensional applications.
Systems for cancelling repetitive noise and vibration, for example, Chaplin U.S. Pat. Nos. 4,153,815 and 4,417,098, describe the use of a synchronizing timing signal to provide selective cancellation of repetitive noise or vibration. Additionally a controller, actuator and error sensor are used. The method presented by Chaplin in these patents divides the noise or vibration period into a number of intervals and adjusts the amplitude of the cancelling signal within each interval in response to the sign or amplitude of the error sensor within the same or a delayed interval.
In U.S. Pat. No. 4,490,841, Chaplin describes the use of Fourier transforms to process signals in the frequency domain. While this method might be used for random signals, processing time requirements generally limit its application to repetitive signals.
These systems use expensive or complicated filters and do not account for variable delays in the system.
Cancellation of unwanted components within electronic signals generally is applied to communication signals. Rennick et al. in U.S. Pat. No. 4,232,381, use a commutating filter synchronized to the rotation of an engine to cancel self-generated engine noise within an electronic circuit. The level of the cancelling signal is adjusted manually and no method is provided to adapt to phase shifts or varying amplitudes of different harmonics.
Garconnat et al. in U.S. Pat. No. 4,594,694 use two sensors, one sensing both the wanted and unwanted signals and the other sensing only the unwanted signals. Narrow band filters or Fourier transforms are used to eliminate the unwanted signals from the combined signal.
Widrow in "Adaptive Noise Cancelling Principles and Applications", Proceedings of the IEEE, Vol. 63, No. 12, December, 1975, describes two forms of active adaptive cancellers. The first, as illustrated in FIG. 1, uses a multi-tap adaptive FIR filter with a reference signal correlated with the noise to be cancelled. The reference signal is required to be within 90.degree. in phase of the error signal. Consequently, the reference signal used by the adapter itself often requires filtering; the resulting approach is referred to as the "filtered-x algorithm."
The second form described by Widrow, as illustrated in FIG. 2, provides a single frequency notch filter and requires only two single tap filters. Again, a reference signal correlated with the noise is used and is phase shifted 90.degree. for one of the filter. Glover, in "Adaptive Noise Cancelling of Sinusoidal Interferences," Stanford University, Stanford, California, May 1975, Ph.D. dissertation, extended this technique to multiple frequencies.
Thus it is an object of the present invention to provide an improved selective active cancellation system for repetitive phenomena.
Another object is to provide an active cancellation system using relatively inexpensive and uncomplicated filters in combination with external synchronized timing.
A still further object of the present invention is to provide an improved rate of adaptation to changes in the level or frequency of the repetitive phenomena.
A still even further object of the present invention is to provide a cancellation sYstem for rePetitive phenomena which accounts for variations in processing time and environmental produced delays.
A still even further object of the present invention is to provide selective cancellation of unwanted signal components and automatic adaptation to the levels and phases of the signal components utilizing synchronization timing signals and a single residual sensor.
A still even further object of the present invention is to Provide for adaptation of the system to maintain an appropriate Phase relationship without the use of external reference signals.
These and other objects are obtained by providing a processor which receives phenomena input signals and timing input signals representing the phenomena to be cancelled and the repetition rate of the phenomena respectively, and which includes inexpensive and simple adaptive filters for generating a cancellation signal by adapting its filtering characteristics as a function of the sum of the signal, and a phase circuit for maintaining the adapting of the filtering characteristics within a 90.degree. phase of the phenomena signal. The adaptive filter includes for each frequency to be cancelled, be it a single frequency and its harmonics or a plurality of fundamental frequencies, a sine and cosine generator responsive to the timing signal and providing inputs to first and second adaptive filters whose outputs are summed to provide the cancellation signal. First and second adaptors adapt the first and second filter weights of the first and second filters as a function of the sensed phenomena signal which represents the residual phenomena and a signal received from the phase circuit so that the first and second adapter operates within 90.degree. phase of the phenomena signal.
The phase circuit measures the delay between providing the cancellation signal and receipt of the Phenomena signal and adjusts the phase as a function of the measured delay. The Phase circuit also takes into account and adjusts the phase as a function of the processing delay of the processor. The phase circuit includes a test signal generator for generating a test signal which is combined with the cancellation signal and provided into the area to be monitored. A third adaptive filter is provided for receiving the test signal and providing a filtered signal. A difference is taken of the filtered signal with the phenomena residual signal. An adapter adapts the third filter weights as a function of the difference signal and a delayed test signal. The third filter weights represent the delay time of the system and are used to provide the appropriate phase correction.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.